Piezoelectric wafers have been used for many decades as frequency control elements in radio communication devices. Typically, individual piezoelectric wafers are fabricated from larger single crystal piezoelectric bars which have been grown synthetically in an autoclave. The general methods of growing piezoelectric bars are well known in the art, and typically include exposing a seed crystal, under high temperature and pressure, to a solution containing substantial quantities of a soluble alkali compound, such as sodium chloride, sodium carbonate, sodium bicarbonate, or sodium hydroxide. The solution is maintained in contact with a supply material of silica so that the solution becomes supersaturated with silica. Crystal growth takes place by the transfer of silica from the supply material, through the solution or fluid, to the quartz seed. Other methods have also been employed with success in which the crystal seed is exposed to other suitable predetermined chemical and physical environments which promote crystalline growth on the seed to produce a piezoelectric crystalline material bar from which individual crystal elements may be fabricated.
Those skilled in the art recognize that quartz is one of the types of crystals which primarily grow elongated in one direction, in the case of quartz, along the Z crystal axis. A fully grown quartz crystal, between its ends, tends to be in the shape of an elongated prism bounded by a set of six faces, known as the primary faces, which extend in the direction of natural elongation, or Z-axis, of the crystal. Typically, at either end the crystal is terminated by three intersecting faces, known as the major rhombohedral faces, which produce tapered end caps on the crystal. In addition, minor rhombohedral faces may be located between the major rhombohedral faces and the primary faces. There may appear additional faces of lessor proportions, but these are not important for the purposes of understanding the invention.
The rate of growth of each of the faces on a quartz bar differs dramatically. Generally, growth in the Z-direction is fastest, followed by the X-directions for synthetically grown bars, the minor rhombohedral faces, the major rhombohedral faces and lastly the primary faces. This growth rate differentiation becomes a major consideration for quartz crystal bar growth. Prior art efforts have grown bars from seeds that were perpendicular to the Z-axis or parallel to a minor rhombohedral face, with the expectation that faster growth results in a cheaper product. However, other pertinent factors such as waste and factory capacity have not been considered in this regard. For popular wafer angles, such as the AT-cut for bulk acoustic wave devices and the ST-cut for surface acoustic wave devices, there can be a considerable amount of waste involved in processing.
An important goal in the manufacture of quartz bars is to obtain maximum output from a factory. This may be accomplished through both increased yield and increased utilization of existing capacity. One approach to this problem has been to grow bars as fast as possible. However, this does not consider waste and capacity problems. Another approach has been to increase the size of wafers produced, thereby increasing the number of usable devices per crystal bar. However, this results in autoclave runs of relatively long duration, more waste, and requirements for more growth space in the autoclave. Another approach is to increase the number of factory autoclaves, but this incurs heavy capital costs.
There is a need for ST-cut and AT-cut quartz crystals that can be produced more quickly in existing facilities through more efficient utilization of materials and equipment and reduction in waste. In this regard, there is also a need to provide an efficient technique to regenerate seed stock for continued processing without diminishing production capacity. In addition, it is desirable to provide wafers as large as possible from a single crystal without those wafers containing a wasteful intervening seed portion.